1. Field of the Invention
The present invention relates generally to a wireless communication system, and in particular, to an apparatus and method for estimating a symbol timing offset in an Orthogonal Frequency Division Multiplexing (OFDM) communication system.
2. Description of the Related Art
In the 4th Generation (4G) communication system, which is the next generation communication system, research is being conducted to provide users with high-speed services having various Qualities-of-Service (QoS). Particularly, in the 4G communication system, research is being carried out to support high-speed services while guaranteeing mobility and QoS for a Broadband Wireless Access (BWA) communication system, such as a wireless Local Area Network (LAN) system and a wireless Metropolitan Area Network (MAN) system.
Therefore, in the 4G communication system, Orthogonal Frequency Division Multiplexing (OFDM) is being studied as a scheme suitable for high-speed data transmission in wire/wireless channels. OFDM, a scheme for transmitting data using multiple carriers, is a kind of Multi Carrier Modulation (MCM) that converts a serial input symbol stream into parallel symbol streams and modulates them with multiple orthogonal sub-carriers before transmission.
The 4G communication system needs broadband-spectrum resources in order to provide high-speed, high-quality wireless multimedia services. However, the use of the broadband-spectrum resources causes a considerable fading effect in the wireless transmission path due to multipath propagation, and also causes a frequency selective fading effect even in transmission bands. Therefore, for the high-speed wireless multimedia services, OFDM, which is robust against the frequency selective fading compared with Code Division Multiple Access (CDMA), has a higher gain. Therefore, OFDM is popularly used for the 4G communication system.
The wireless channel environment in a mobile communication system, unlike the wired channel environment, suffers inevitable errors occurring due to several factors such as multipath interference, shadowing, wave attenuation, time-varying noise, Inter-Symbol Interference (ISI) caused by delay spread, frequency selective fading, and the like, thereby causing a loss of information data. The loss of information data causes considerable distortion of actual transmission signals, thereby reducing the entire performance of the mobile communication system.
In order to overcome the ISI and the frequency selective fading, the OFDM communication system transmits OFDM symbols by inserting Guard Intervals (GI) as well as using multiple orthogonal sub-carriers. A method for inserting the guard interval is classified into a Cyclic Prefix scheme of copying last specific samples of an OFDM symbol in the time domain and inserting the samples into an effective OFDM symbol, and a Cyclic Postfix scheme of copying first specific samples of an OFDM symbol in the time domain and inserting the samples into an effective OFDM symbol.
As described above, the OFDM communication system reduces the ISI effect using the guard intervals. That is, the OFDM communication system minimizes an effect of wireless channels using the guard intervals.
However, in the OFDM communication system, an effect of symbol timing is considerable in high-order modulation of the sub-carriers, for example, in 64-ary Quadrature Amplitude Modulation (64 QAM) or 256 QAM, and in this situation, the ISI effect remains.